The subject matter disclosed herein relates to multi-energy X-ray imaging systems and, more particularly, to systems and methods for producing increased mean energy separation of X-ray spectra delivered in such systems.
In modern medicine, medical professionals routinely desire to conduct patient imaging examinations to assess the internal condition of a patient in a non-invasive manner. For typical single-energy computed tomography (CT) imaging, the resulting X-ray images are largely a representation of the average density of each analyzed voxel based upon the attenuation of X-rays emitted by the X-ray source by a patient or object, and detected by an X-ray detector. However, for multi-energy X-ray imaging, a greater amount of information may be gleaned for each voxel. For example, in a dual-energy X-ray imaging system, X-rays of two different spectra are applied to the patient or object; high-energy X-ray photons are generally attenuated substantially less by patient tissue than low-energy X-ray photons. In order to reconstruct multi-energy CT projection data, the underlying physical effects of the X-ray interactions with matter, namely, the Compton scattering effects and photoelectric effects, are utilized in a process known as material decomposition (MD), as is known in the art.
During multi-energy CT data acquisition, a multi-energy X-ray source may be used to provide the X-rays having different energy spectra and may be capable of quickly switching between emitting an X-ray spectrum having one average energy to emitting another X-ray spectrum having a different average energy. Such sources are typically called fast-switching kVp (peak operating voltage) sources because the operating voltage to the source is switched quickly between high and low potentials on subsequent CT projection data acquisitions to enable acquisition of projection data closely correlated in both time and space. However, the rapid kVp switching requirements from a single X-ray source limits the ability to employ dynamic beam filtration schemes between the high- and low-energy projection data acquisitions, e.g. rapidly switching a filter out of and into the X-ray beam during low-energy and high-energy acquisitions, respectively. Dynamic filtering schemes are employed to selectively filter the high-energy X-ray spectrum to improve the mean energy separation between the low-energy and high-energy spectra. The mean energy of a spectrum is the energy level of an average photon in the spectrum; it is computed by summing all energies in a given X-ray spectrum after weighting each energy level by the percentage of photons at that specific energy. Thus, without dynamic filtration, there is significant spectral overlap in the low-energy and high-energy projection data acquisitions, limiting the mean energy separation between the two projection data acquisitions. Energy separation is desirable in multi-energy images because it improves the independence of the measurements and enhances the material decomposition process, thereby improving the clinical usefulness of the reconstructed multi-energy images. As known in the art, multi-energy images comprise basis material images, monochromatic images (images reconstructed as if the applied X-ray spectrum consisted of a single energy), or images reconstructed directly from an applied energy spectrum. Accordingly, there exists a need for systems that enable multi-energy X-ray imaging with a fast-switching kVp source and dynamic filtering schemes in order to increase the mean energy separation of the applied X-ray spectra.